1. Technical Field
An embodiment of the invention relates to a discharge circuit comprising an output circuit having one output connected to an electrical load, for example of a capacitive type, to absorb a discharge current given by the load when a logic signal commands a discharge of the load. An embodiment of the invention is especially valuable for the making of power output stages for the control of high-voltage circuits such as, for example, plasma display panels.
A plasma display panel is a matrix type screen or panel formed by cells positioned at the intersections of rows and columns. A cell comprises a cavity filled with a rare gas, two control electrodes, and a deposit of red, green or blue phosphorus. To create a light dot on the screen in using a given cell, a potential difference is applied between the control electrodes of this cell, so as to activate an ionization of the gas. This ionization is accompanied by an emission of ultraviolet rays. The creation of the light dot is obtained by the excitation of the deposited phosphorus by the emitted rays.
The cells are classically activated to create images by means of logic circuits producing control signals. The logic states of these signals determine the cells that are commanded to produce a light dot and the cells that are commanded not to produce any light. The logic circuits are generally powered at low voltage, for example voltage of 5V or less. This voltage is typically not sufficient to directly drive the electrodes of the cells. Between the logic circuits and the cells to be controlled, power output stages are therefore used to convert the low-voltage control signals into high-voltage control signals.
The ionization of the gas of the cavities typically necessitates the application of high potentials to the control electrodes, for example, about 100 V. Furthermore, it may be necessary to be able to provide the electrodes with high currents, in the range of several tens of milliamperes (and correlatively to be able to receive these currents from these electrodes). Indeed, the electrodes may be represented schematically by equivalent capacitors having relatively high capacitance values of about 100 picofarads. The controlling of the electrodes may be, therefore, equivalent to the control implemented for charging or discharging a capacitor.
In plasma display panels, it is generally desired to obtain signals (current, voltage signals) that have fast edges (i.e., rise and fall times). This represents, for example, charging or discharging times of about 100 nanoseconds. Given the high potential to be attained and the size of the capacitive charge, this entails the assumption that it is possible to provide very high charging currents and absorb very high discharge currents that could go up to about 100 milliamperes in one example.
2. Description of the Prior Art
FIG. 1 illustrates an example of a classical embodiment of an output stage used to control an electrode schematically represented by a capacitor CLoad. The stage has a potential step-up circuit (i.e., a level shifter) 10, and an output circuit 20. The circuit 10 has the function of converting a low-voltage logic control signal IN into a high-voltage logic control signal INP varying between a low voltage such as 0 V and a high-voltage VPP, and following the variations of the signal IN. The circuit 20 has a P type transistor T21 for charging the capacitor CLoad and an N type transistor T22 for discharging this capacitor CLoad. The transistor T21 is driven by the high-voltage control signal INP: when it is on, the transistor T21 gives the load CLoad a charging current, which will give rise to an increase in the potential OUT up to VPP. The transistor T22 is driven by the low-voltage control signal IN: when it is on, the transistor gives the load CLoad a discharge current proportional to the potential of the signal IN and the potential OUT decreases along a slope proportional to the discharging current. The circuit of FIG. 1 is described at greater length in French patent FR 2,763,735.
Apart from the relatively large sizes of the transistor T22 and the transistor T21, one drawback of the circuit of FIG. 1 is the risk of simultaneous conduction of the transistors T21, T22. This risk entails major dissipation, given the values of voltage and current present in the circuit.
Yet another drawback of the circuit of FIG. 1 lies in the electromagnetic disturbances that it causes in the cells of the plasma display panel. Indeed, as seen here above, the voltage and currents brought into play are substantial and they vary in substantial proportions over very short periods of time during changes in state of the control signal. These sudden, high-amplitude variations in the voltages and currents may give rise to electromagnetic radiation that disturbs some or all of the cells of the plasma display panel.
French patent 2,763,735 also proposes another structure of an output stage that reduces the surface area needed for the charging transistor T21 and that prevents the simultaneous conduction of the transistors T21, and T22 during changes in the state of the control signal. To this end, the charging transistor T21 is replaced by a charging circuit comprising an N type transistor driven by a control circuit comprising the transistor and one diode; the charging transistor has a mode of operation similar to that of the PMOS type transistor which it replaces. But even though the size of the N type transistor is reduced relative to that of the transistor T21 of FIG. 1, the total size of the charging circuit is appreciably greater. This is because in this structure, the discharging transistor is a DMOS type transistor, which has the advantage of having particularly short power-off and power-on times, but is, on the contrary, far bulkier and furthermore requires control inverters (to introduce delays in the control signals, the delays being necessary to prevent the charging circuit and the DMOS transistor from being powered on simultaneously), which further increases the total size of the output circuit. Furthermore this circuit structure does not provide any solution to limiting the electromagnetic disturbances generated by the changes in the state of the control signal IN.